These compositions are generally used for coming in contact with electronic components as in applications of the “packaging for electronics” type or of the support plate type for handling electronic components. In these applications, it is desirable to be able to easily dissipate the electrostatic charges in order not to damage the electronic components which are in contact with the composition. The composition must therefore have a volume resistivity less than approximately 1011 Ω·cm, without however being totally conductive, since a possible discharge could then be violent and damage the components. The introduction of carbon block in sufficient quantity in a polymer makes it possible to lower its volume resistivity. The intermediate resistivities between 105 and 1011 Ω·cm are generally difficult to attain because of the percolation curve of carbon black. In effect, in this domain of resistivities, the resistivity varies greatly with the percentage of carbon black, and the latter must therefore be measured out exactly. In a general manner, it can be considered that for a given carbon black/polymer pair, there is a percolation curve whose course is that of FIG. 1.
The introduction of carbon black in a polymer is generally difficult, especially in a material of the polystyrene type which poorly accepts solid fillers, which is generally expressed by an insufficient conductivity and a loss of mechanical properties (because of the formation of agglomerates of carbon black and therefore also of zones free of carbon black and which are poor conductors). The dispersion of the carbon black must therefore be sufficient so that the material is conductive and keeps suitable mechanical properties. Moreover, said dispersion must be stable during transformation, particularly with regard to the extensive shearing encountered during this transformation such as injection, under penalty of loss of conductivity and of mechanical properties. In effect, excessive shearing brings about deterioration of the particles of carbon black which lose their conductivity.
Furthermore, it is desirable for the material filled with carbon black to keep a suitable impact resistance, which is known to be difficult in the case of vinylaromatic polymers.